Dependence on the choice of the thin- or thick-disc stars: correlation between the bar strength, S_bar and the strength of the quadrupole, S_quadrupole (top row), and correlation between the bar length, R_bar and the length of the quadrupole, R_quadrupole (bottom row), computed using thin-disc particles (left panels) and thick-disc particles (right panels), for all thin+thick models and the sim6 model (see the legend). The black dash line denotes the best-fit straight line (of the form Y = AX + B), while the grey shaded region denotes the 5-σ scatter around the best-fit line. Regardless of the choice of thin or thick-disc stellar particles, the length and strength of the quadrupole remain strongly correlated with the length and the strength of the bar.